1. Field of the Disclosure
The present invention generally relates to a photomask, and more particularly, to a method of correcting a pattern critical dimension (CD) of a photomask.
2. Brief Description of Related Technology
Photomasks are used to form a pattern on a semiconductor substrate during the manufacture of a semiconductor device. Photolithography is used to transfer a pattern on the photomask onto a wafer. As semiconductor devices become more highly integrated, the size of the patterns on a photomask becomes smaller and smaller.
A correction process is performed to correct the CD of a photomask pattern and to improve the CD accuracy of the photomask pattern. Specifically, a phase shift layer is formed on a transparent substrate, and a light blocking pattern that selectively exposes the phase shift layer is formed on the phase shift layer. A CD of the light blocking pattern is measured. The CD of the light blocking pattern is corrected by etching the light blocking pattern using a separate correction mask. The exposed phase shift layer is patterned using the corrected light blocking pattern as an etch mask.
The separate correction mask is formed by applying a resist coating to the transparent substrate having the phase shift layer and the light blocking pattern, and performing exposure and development processes. The correction mask is formed using a typical resist layer, for example, a positive resist layer. During the development process, the portion of the positive resist layer exposed to an electron beam is removed by a developing solution. The portion of the positive resist layer not exposed to the electron beam remains and becomes a mask pattern for the correction process.
When the separate correction mask is formed, foreign materials such as particles may adhere to the resist layer. These foreign materials prevent a portion of the resist layer that should be exposed from being exposed and removed during the development process. As a result, resist residue may remain on the surface of the phase shift layer. The resist residue may cause a bridging defect of the phase shift patterns when the phase shift layer is etched. The bridging defect may degrade the quality of the photomask and cause a patterning defect during the wafer exposure process.